Aluminum metal materials are well known to have excellent corrosion resistance. However, electrodeposition of aluminum from an aqueous solution is difficult, because aluminum has a high affinity for oxygen and has a reduction potential lower than hydrogen. For this reason, electroplating of aluminum has been conventionally carried out by using an organic solvent-based plating bath or a high-temperature molten salt bath. Here, typical examples of the organic solvent-based plating bath include those obtained by dissolving AlCl3 together with LiAlH4 or LiH in ether or in tetrahydrofuran, and a toluene solution of NaF.2Al(CH2H5)3. However, these baths have a problem of poor handleability, because the baths have risk of explosion upon contact with the air or water.
In this respect, a mixture molten salt bath containing an aluminum halide and an alkylpyridinium halide is proposed as a bath having no risk of explosion (Japanese Patent Application Publication No. Sho 62-70592). However, plating using this plating bath has the following problem. Specifically, the plating is poor in flatness and smoothness due to non-uniform electrodeposition. Especially when the film thickness is increased, or when a high current density is employed, dendritic deposits or black deposits are formed in a high-current density portion, and the deposits easily fall off. In addition, the throwing power of the plating is also so poor that the obtained plating film does not have an expected rust-prevention performance in a salt spray test or the like without a chromate treatment using hexavalent chromium. In this respect, as a method for solving the problems of the molten salt bath, a method has been proposed which uses benzene, toluene, xylene, or the like for dilution. However, it is not preferable to use a large amount of benzene, toluene, or xylene, because of their adverse effects on the human body, and risk of ignition due to their low flash points. This hinders the industrial application of the Al plating.